The use of toner transfer intermediates has been suggested for a number of reasons in electrophotography including simplified receiving sheet handling, doing single pass duplexing, saving wear on photoconductors and superposition of images, e.g., to form multi-color images. Typically, a toner image is created on a photoconductive member electrophotgraphically and is transferred by conventional, electrical field assisted transfer to an intermediate roller or web. For example, a negatively charged toner image is transferred from a photoconductor having a grounded backing electrode to an intermediate web or roller biased to a strong positive polarity. The toner image is then transferred from the intermediate member to a receiving sheet under the influence of a second electric field which can be created without changing the field on the intermediate member by placing a roller (or corona) behind the receiving sheet biased still stronger in a positive direction.
Although other reasons mentioned above for using intermediate transfer are still valid, it appears the most desirable use of it in the future may be for creating multi-color images. When an intermediate transfer member is used, two, three, or four separate images of different color can be transferred in registration to the intermediate to create a multi-color image and then the multi-color image can be transferred in one step to the receiving sheet. This system has a number of advantages over the more conventional approach to making multi-color images in which the receiving sheet is secured to the periphery of a roller and rotated repeatedly into transfer relation with the photoconductor to receive the color images directly. Probably the most important advantage is that the receiving sheet itself does not have to be attached to a roller. This has turned out to be a source of misregistration of images as well as complexity in apparatus. Other advantages associated with wear and tear on the photoconductive member and a straight and simple receiving sheet path are also important.
As color electrophotography improves, especially electrophotographic color printing, higher and higher resolutions are desired. In order to obtain higher resolution in color electrophotography, fine toners are necessary. Toners less than 20 microns, and especially toners less than 10 microns in size, give substantially improved resolution in color imaging with high quality equipment.
Unfortunately, fine toners are more difficult to transfer electrostatically than are traditional coarse toners. This is a problem in conventional electrophotography utilizing a single transfer with fine toner particles. It is considerably more of a problem using intermediate transfer members where two transfers are necessary.
Many transfer materials have been suggested for intermediate transfer systems. The most common are relatively soft materials, such as silicone rubber, polyurethane, or fluroelastomers; see, for example, U.S. Pat. Nos. 3,893,761; 4,453,820; 3,923,392; 4,455,079; 4,453,820; 4,068,937; or 3,697,171.
U.S. Pat. No. 4,430,412 is typical of a number of patents in which the first transfer is made with or without the benefit of an electrical field by choice of materials and the second transfer to the receiving sheet is aided by heating the toner to its softening point which both aids the transfer and provides an at least partially fixed image on the receiving sheet. This patent suggests that certain silicone rubber materials are soft enough to "seize" the toner from the photoconductor, but still permit transfer with the aid of the heat at the second transfer. Although the materials suggested in this patent may work well in a system which utilizes heat at the second transfer, when used with dry materials in the absence of heat and utilizing electrostatics for both transfers, they are effective to receive the image from most photoconductive members, but are too soft to pass it well to the usual receiving sheet. The role of soft materials on adhesion is discussed in a paper by D. S. Rimai, L. P. DeMejo and R. C. Bowen, J. Appl. Phys. 66, 3574-3578 (1989). In brief, the soft substrate allows the particles to embed, thereby increasing the force of adhesion and making removal difficult.
When transferring toners having a mean particle size less than 20 microns and using electrostatics at both transfers, a number of transfer artifacts occur. For example, a well known artifact called "hollow character" causes insufficient transfer in the middle of high density toned areas, e.g., in alphanumerics. Another artifact, "halo" is experienced when toner fails to transfer next to a dense portion of an image. Use of the materials suggested in the prior art tends to give these artifacts and others when using two electrostatic transfer steps. These problems cannot be eliminated merely by increase of the transfer field, since that expedient is limited by electrical breakdown.
Studies have been done of the forces causing adherence of toner to photoconductive and other surfaces as toner particles become smaller. These studies indicate that forces such as van der Waals forces holding toner to the surface of a photoconductive element have greater holding effect compared to electrostatic image forces as toner particles become smaller. For example, it is believed that, as toner particle size is reduced to 10 microns, the electrostatic force of the electrostatic image may be less than 10% of the total forces holding toner to the surface. See Rimai and Chowdry, U.S. Pat. No. 4,737,433. See, also, Dessauer and Clark, Xerography And Related Processes, page 393, Focal Press (NY), N. S. Goel and P. R. Spencer, Polym. Sci. Technol. 9B, pp 763-827 (1975).